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Jan 17, 2001CSCI {4,6}900: Ubiquitous Computing3 Central Server based A central server provides service –Reliability and fault tolerance If server shuts down, then no service –Scalability Performance bottle neck E.g. if everyone accesses Microsoft.com from the east coast (new release of web browser), accesses to Yahoo.com in California might be slow because we share the same link from east coast till Utah (say) –Easy to deploy, administer

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Jan 17, 2001CSCI {4,6}900: Ubiquitous Computing11 DNS Name Server Hierarchy.edu duke uga cs ucns chem... DNS servers are organized into a hierarchy that mirrors the name space. Specific servers are designated as authoritative for portions of the name space. com gov org net firm shop arts web us fr Root servers list servers for every TLD. Subdomains correspond to organizational (admininstrative) boundaries, which are not necessarily geographical. Servers may delegate management of subdomains to child name servers. Parents refer subdomain queries to their children. Servers are bootstrapped with pointers to selected peer and parent servers. Resolvers are bootstrapped with pointers to one or more local servers; they issue recursive queries. Source: Jeff Chase

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Jan 17, 2001CSCI {4,6}900: Ubiquitous Computing12 Peer-to-peer systems Decentralized, no "server" Robust – no single point of failure "Will perform work for others since they will work for us" computing Can scale up Locating resources harder E.g. napster (has a central directory server) gnutella

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Jan 17, 2001CSCI {4,6}900: Ubiquitous Computing13 Gnutella Queries issued by a servant at a given node propagate out to neighbor nodes The neighbors propage the query to their neighbors, and so on, for a given number of hops. Depending on where a user's query is first issued, it may or may not reach a node that has the file sought by the user.

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Jan 17, 2001CSCI {4,6}900: Ubiquitous Computing14

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Jan 17, 2001CSCI {4,6}900: Ubiquitous Computing15 Scalability The scalability of a Gnutella network to accommodate more users performing more searches is limited by the lowest bandwidth links prevalent within the network For dial-up users it is 10 requests per second and has been reached Bottleneck Link